1. Field of Invention
The present invention generally relates to an electrochemical device and its fabricating method. More particularly, the present invention relates to a method of forming an electrode including a porous conductive fabric layer and an electrochemical catalyst layer comprised of polymer-capped nanoclusters, and to an electrochemical device including the electrode made thereby.
2. Description of Related Art
Recently, dye-sensitized solar cells (DSSC) have attracted attention as a potentially low-cost energy device. Typically, a DSSC consists of a dye-sensitized nanocrystalline semiconductor film on an Indium-tin oxide (ITO) or fluorine-doped tin oxide (FTO) glass as the photo-anode, a platinized counter electrode acting as the cathode, and iodide/tri-iodide redox couples in a proper mediator as the electrolyte. The working principle of a DSSC is summarized in the five steps below, as shown in FIG. 1. (1) Photo-excitation on dye molecules induces charge separation (see arrow 1). (2) Charge (electron) is injected into the conduction band of mesoporous titanium dioxide (TiO2). (3) Charge passes through the outer circuit via the electronic load (see arrow 2). (4) Dye reduces to ground state by redox couples in the electrolyte (see arrow 3). (5) Redox couples reduce on counter electrode by the charge coming from the outer circuit (see arrow 4).
In a DSSC, the counter electrode functions as a reduction reaction site expressed as following:I3−+2e−→I−.
This reduction reaction is vital since iodide ions are responsible for the regeneration of oxidized dye molecules. Once the dye regeneration rate cannot catch up with the dye oxidation rate (i.e. electron injection from dye molecules to the conduction band of TiO2), the conversion efficiency is reduced and the DSSC deteriorates because iodine crystals may be deposited on the surface of the counter electrode.
In the prior arts, the naked ITO or FTO glass shows extremely slow kinetics of tri-iodide reduction in organic solvents. In order to minimize the over-potential, catalyst material is applied to the ITO or FTO glass to speed up the reaction.
So far, platinum (Pt) has been used almost exclusively as the catalyst material. Depending on the cost and efficiency, there are many methods to form a thin layer of Pt. Sputtering is a common method. This platinized electrode exhibits fair performance. However, sputtering requires an ultra-high vacuum environment so that the cost of ownership remains high.
Papageorgiou et al. have developed a method called “thermal cluster platinum catalyst” (Coord. Chem. Rev., 2004, 248, pp 1421). This method provides low Pt loading (about 2˜10 μg/cm2), superior kinetic performance (charge-transfer resistance, RCT<0.1 Ωcm2) and mechanical stability with respect to conventional platinum deposition methods like sputtering or electrochemical deposition. Wang et al. (Surf. Interface Anal., 2004, 36, pp 1437) have studied the stability of thermal cluster Pt (TCP) electrode with X-ray photoelectron spectroscopy and found that the electrochemical catalytic performance of TCP may be reduced slightly due to adsorbed iodide on TCP's surface. The electrochemical catalytic performance can be regenerated with re-heating treatment, but this method requires heating of up to 380° C. It consumes power, and is not suitable for mass production.
Other materials such as carbon and conducting polymers are also proposed to be the catalyst for tri-iodide reaction in DSSC. These new materials usually need to be deposited as thicker films on the substrate to obtain acceptable catalytic effect, and are still being developed.
Hence, lots of research on DSSC and technologies relative to DSSC address lower costs of materials and higher performance of cells.